소스 또는 드레인 전극과 채널 층 사이의 접촉 저항을 줄이기 위해 자외선 조사에 따른 비정질 InGaZnO₄박막 트랜지스터의 전기적인 특성 개선

Abstract

Amorphous InGaZnO4 thin film transistors (a-IGZO TFTs) offer an attractive alternative to amorphous Si TFTs because of their high mobilities (>10cm2/Vs) and low process temperature (<250℃) compared to those of amorphous Si TFTs. Moreover, Conventional polycrystalline Si TFTs suffer from the non-uniformity of their mobility and threshold voltage, due to the existence of grain boundaries. Thus, it is evident that amorphous InGaZnO4 thin film transistors are critical for fabrication of competitive AMOLEDs.

Recently, even though Hosono et al. reported high performance TFTs with amorphous InGaZnO4 channel showed high mobility(>10cm2/Vs) and excellent sub-threshold swing (0.2V/decade) even in amorphous phase, it needs to enhance the better electrical properties to apply to competitive AMOLEDs and TFT-LCDs.

In order to further improve the performance of the a-IGZO TFTs, additional attention should be given to reduction of contact resistance between the source or drain electrode and channel layer because the major loss of the device performance is often due to the presence of high resistance metal-semiconductor contacts. However, only few studies designed to improve the contact resistance between a metal electrode and a-IGZO channel layer.

In this study, we attempted to reduce the contact resistance between the source or drain (Mo) electrode and channel layer (a-IGZO) by using UV treatment. Before conducting this study, we got interested results form UV treatment on a-IGZO thin films. The net electron carrier concentration of the a-IGZO thin films dramatically increased and its resistivity decreased.

From these facts, we compared between a-IGZO TFTs with UV treatment and as-deposited a-IGZO TFTs in terms of contact resistance and electrical properties.

In terms of contact resistance, when a-IGZO channel layer is operating, there are two components of contact resistances. They are the resistance of the source or drain electrode / a-IGZO interface (RInjection) and the access resistance (RAccess) of a-IGZO thin film itself from the metal contact to the channel layer. Thus, through UV treatment for 2 hours, over 1 order of the contact resistance between source or drain electrode and a-IGZO channel layer could be reduced.

In terms of electrical properties, without UV treatment, a-IGZO TFTs with W/L=40/50 ㎛ exhibited a moderate field effect mobility (&micro; FE) of 2.4 cm2/Vs, on/off ratio of 6.3×106, subthreshold swing (SS) of 0.3 V/decade and threshold voltage of 11 V, but the device performance of a-IGZO TFTs was significantly improved by UV treatment. As expected, an excellent on/off ratio of 2.2×108, subthreshold swing (SS) of 0.1 V/decade and threshold voltage of 8 V, as well as a high &micro; FE of 6.1 cm2/Vs, were achieved for the UV treated a-IGZO TFTs.

In conclusion, a high performance a-IGZO TFTs have been successfully fabricated and satisfied a requirement of AMOLED back plane as its contact resistance between source or drain electrode and channel layer was reduced using UV treatment.